1. Field of the Invention
This invention relates to gasification of carbonaceous materials and more particularly to a method for separation and cooling of ash from fluidized bed gasifiers.
2. Description of the Prior Art
In reactors for the gasification of carbonaceous materials, such as coal, a combustible product gas is produced as well as solid waste products such as agglomerated ash. In a typical fluidized bed gasifier, coal particles are pneumatically transported by a gas into the hot gasifier. Process mediums such as steam, coal in particle form, and a gaseous source of oxygen, such as air or pure oxygen, as well as, perhaps, a clean recycled product gas are injected through a nozzle. This process results in fluidization of the coal particles in a bed above the nozzle. Further, the injection of coal and oxygen into the hot gasifier results in combustion of a portion of the coal, and the heat thereby released maintains the temperature in the gasifier. As the noncombusted coal particles are heated, rapid evaporation of volatiles in the coal, called devolatilization, occurs. The average temperature within the vessel typically runs between 1600.degree. F. and 2000.degree. F. or higher and this high temperature ensures that the products of devolatilization, such as tars and oils, etc., are broken down, or cracked, and gasified to form methane, carbon monoxide and hydrogen. As the coal continues to heat, devolatilization is completed and particles of coal become pieces predominantly of ungasified carbon, or char. As this char circulates throughout the fluidized bed, the carbon in the char is gradually consumed by combustion and gasification, leaving particles that have a high ash content. These ash-rich particles contain mineral compounds and eutectics that melt at temperatures of between 1000.degree. F. to 2000.degree. F. and typically consist of compounds of any or all of S, Fe, Na, Al, K and Si, which compounds are typically denser than carbon compounds. These liquid compounds within the particles extrude through pores to the surfaces where they cause the particles to stick to each other, or agglomerate. In this way, ash agglomerates are formed that are larger and denser than the particles of char in the bed. As their density and size increases, the fluidized bed is unable to support them, and the ash agglomerates defluidize. It is then necessary to remove these ash agglomerates from the vessel.
This process of combustion, gasification and ash agglomeration is not a particularly rapid or complete process. Typically, coal particles pneumatically injected into the gasification vessel are traveling at a fairly significant velocity at the nozzle outlet. These particles may travel quickly through a combustion flame and be only partially combusted and gasified prior to melting of the mineral compounds and eutectics. As a consequence, it is desirable to recirculate these particles back through the zone in which combustion is taking place.
One method of recirculation may be to entrain and discharge all the particles with the product gas, separate the product gas from the particles in a device external to the gasifier vessel, then recirculate these particles back into the vessel. This is not a particularly efficient method of recirculation.
A more efficient means of recirculation would be an internal recirculation means which would result in recirculation of the particles back through the combustion zone without leaving the gasifier vessel. One embodiment of this means involves distributing a gas into the gasification vessel by means of a refractory brick assembly having gas distribution outlets. This design is inadequate for several reasons. The gas may bypass the gas distribution outlets through micro-cracks and fissures in the refractory brick causing non-uniform distribution. The nature of refractory brick makes the steam distribution outlets difficult to fabricate and properly size, which may cause solids to back-flow into the outlets. Further, the mere introduction of a gas into the periphery of the vessel does not necessarily result in any solid recirculation.
What is needed is an internally contained, plug resistant, solids recirculation apparatus and method which will promote solids recirculation within a fluidized bed gasifier in a uniform pattern, and which will be easily fabricated and installed.
It is also desirable to provide an ash separation means which will allow cooling of the ash prior to withdrawal to minimize fouling of internal gasifier surfaces. It is further desirable to accomplish the above in a manner which discourages perturbations in the dynamics of the fluidized bed.